Recently a new class of semiconductor lasers, designated "quantum cascade" or "QC" lasers, was discovered. See, for instance, U.S. Pat. Nos. 5,457,709, 5,509,025 and 5,570,386, all incorporated herein by reference. See also U.S. patent applications Ser. No. 08/825,286 (filed Mar. 27, 1997) now U.S. Pat. No. 5,978,397 and Ser. No. 08/744,292 (filed Nov. 6, 1996), now U.S. Pat. No. 5,745,516, both incorporated herein by reference.
A "quantum cascade" or "QC" photon source is a unipolar semiconductor device having a multilayer structure that forms an optical waveguide, including a core region of relatively large effective refractive index between confinement regions of relatively small effective refractive index. The core region comprises a multiplicity of nominally identical repeat units, with each repeat unit comprising an active region and a carrier injector region. The active region has a layer structure selected to provide an upper and a lower carrier energy state, and such that a carrier transition from the upper to the lower energy state results in emission of a photon of wavelength .lambda.. The carrier injector region has a layer structure selected to facilitate carrier transport from the lower energy state of the active region of a given repeat unit to the upper energy state of the active region of the adjacent (downstream) repeat unit. A carrier thus undergoes successive transitions from an upper to a lower energy state as the carrier moves through the layer structure, resulting in the creation of a multiplicity of photons of wavelength .lambda.. The photon energy (and thus .lambda.) depends on the structural and compositional details of the repeat units, and possibly also on the applied electric field and/or a distributed feedback element.
Prior art QC photon sources can be designed to emit at wavelengths in a wide spectral region of the infrared, exemplarily 3-13 .mu.m. Such sources (especially lasers) have a variety of potential uses, e.g., trace gas analysis, environmental monitoring, industrial process control, combustion diagnostics, and medical diagnostics. See, for instance, K. Namjou et al., Optics Letters, Vol. 23.p. 219 (1998).
In at least some of the potential uses (as well as in quantum optics studies) it would be desirable to have available a semiconductor photon source that emits at more than one wavelength. Exemplarily, such a source would be extremely useful for those techniques, like differential absorption lidar (DIAL), where light scattering has to be evaluated and compared at two different wavelengths. See M. Sigrist, editor, "Air Monitoring by Spectroscopic Techniques", Wiley, New York (1994). In addition, if the photons that are emitted at the two wavelengths are correlated, then such a source would make it possible to eliminate spontaneous emission noise in measurements that require beating or heterodyning of two laser radiations. See, for instance, M. 0. Scully, Phys. Rev. Letters, Vol. 55, p. 2802 (1985). This application discloses such a photon source.
The above referenced '286 application inter alia discloses a QC laser with divided electrode that can have dual wavelength emission. Such a laser needs three terminals and thus requires more complex circuitry, and the two wavelengths have relatively small separation. Lasing on two or more coupled transitions has previously been observed in dye molecules and HeNe-based gaseous mixtures. See H. Fu et al., Phys. Rev. Letters, Vol. 60, p. 2614 (1988), and H. J. Gerritsen et al., App. Physics Letters, Vol. 4, p. 20 (1964), respectively.